Carrier mobility is the speed of movement of an electron or hole through the channel region of a transistor. This is an important property for high speed transistors. The higher the carrier mobility, the higher the operating frequency of the transistor. The addition of germanium dopant impurities in a silicon substrate increases channel mobility over that of pure silicon.
Conventional silicon transistor gates are formed by thermally oxidizing the silicon substrate. A thermally grown gate oxide is typically formed by exposing the silicon substrate surface to an oxygen containing ambient. The oxygen causes the silicon surface to be partially consumed and converted into the gate oxide. Unfortunately, a stable gate oxide can not be readily formed from a silicon-germanium substrate because germanium becomes unstable at high temperatures. In other words, the germanium begins to diffuse from the silicon at high temperatures, and consequently, does not retain the properties exhibited at low temperatures. Thus, oxide should not be present at the silicon-germanium interface because the germanium bond is not stable under normal operating conditions. The diffused germanium would create interface trap sites in the gate oxide which may severely limit the electrical performance of the transistor.
One approach to forming a stable gate oxide over a silicon-germanium substrate includes depositing a low temperature CVD oxide. However, such an oxide has a resulting undesirable higher surface state density. Another way of forming a stable gate oxide over a silicon-germanium substrate is by reoxidation of a silicon cap layer applied over the silicon-germanium substrate. Using a silicon cap layer results in a buried channel structure with an undesirably large effective gate oxide thickness.
Nonetheless, silicon-germanium transistors are desirable because of the higher operating speeds that can be achieved as compared to the operating speeds of silicon transistors.